Characterization of microstructure and surface properties of hybrid coatings of WC–CoCr prepared by laser heat treatment and high velocity oxygen fuel spraying

Abstract

The microstructure and microhardness of high velocity oxygen fuel-sprayed WC–CoCr coatings were comparatively studied both before and after laser heat treatment of the coatings. Optical microscopy, scanning electron microscopy, X-ray diffraction and microhardness testing were applied to investigate the microstructure, phase composition, porosity and microhardness. The results indicate that WC is still present, and W 2C has appeared, while neither cobalt nor σ-CrCo is detectable. Co 4W 2C has appeared in the high velocity oxygen fuel-sprayed coating after laser heat treatment as compared to the coating before laser treatment. The relative content of the W 2C has not increased with laser treatment, but the laser treatment has essentially eliminated the porosity almost entirely, providing a more homogeneous and densified microstructure. The laser heat treatment has effected the formation of a denser compact coating on the substrate. After laser heat treatment, the thickness of the coating has decreased from 300 μm to 225 μm. This corresponds to an average porosity in the high velocity oxygen fuel-sprayed coating that is approximately five times greater than that in the subsequently laser heat-treated coating. The laser treatment has also resulted in an increased hardness of the coating near the surface, where the average value increased from Hv 0.2 = 1262.4 in the coating before laser heat treatment to Hv 0.2 = 1818.7 after laser heat treatment.